This invention pertains to an engine speed control for a variable speed engine which control is responsive to a demand for increased flow from a pump of a hydrostatic transmission or to either of a plurality of pumps with at least one of the pumps being usable in a hydrostatic transmission as for driving a vehicle and the other pump being a source of fluid for an implement circuit associated with the vehicle.
Many vehicles, particularly off-the-road vehicles, such as earth working equipment, which also have an implement circuit, now use one or more hydrostatic transmissions for driving the traction wheels or tracks of the vehicle. Frequently, the prime mover engine of such vehicle is set to operate at a certain speed and continues to operate at that speed under normal conditions. The vehicle speed under normal conditions can be controlled by controlling the displacement of one of the components of the hydrostatic transmission without adjusting engine speed. An example of such utilization of a hydrostatic transmission is shown in Donald J. MacIntosh et al. U.S. Pat. No. 3,946,560, owned by the assignee of this application, wherein a pair of hydrostatic transmissions are disclosed as usable in a dual path transmission. The pair of hydrostatic transmissions are associated with traction means, such as tracks positioned at opposite sides of the vehicle. The MacIntosh et al. patent discloses that a wide range of vehicle speed can be established by controlling the displacement of the components of the hydrostatic transmissions.
Better fuel economy, as well as lower noise of operation and longer component life, can be achieved if the prime mover engine normally operates at a set speed which is a relatively low working engine speed less than maximum speed. It is known to operate an engine at an idle speed and cause the speed of the engine to increase when there is flow demand from an engine-driven pump. An example of this is a vehicle used by utility companies wherein the vehicle may be parked and the engine idling. When a personnel lift or other device is manipulated, a system causes an increase in the speed of the vehicle engine to provide more flow from a pump supplying fluid to the fluid circuit. Such systems do not relate to the control of engine speed responsive to flow demands of a hydrostatic transmission used in the drive of the vehicle, nor to control of engine speed when any one of a plurality of pumps driven by the engine are required to provide an output flow beyond that which the pump can provide at the lower engine speed.